I. Field
The present disclosure relates generally to processes for the chemocatalytic conversion of an adipic acid substrate to epsilon-caprolactam; more specifically, it relates to chemocatalytic conversion of an adipic acid substrate, preferably adipic acid, to intermediates amenable to cyclization to epsilon caprolactam. The processes involve heterogeneous catalysis in the presence of hydrogen, ammonia, particular heterogeneous catalysts and, preferably, particular solvents.
II. Related Art
Epsilon Caprolactam (hereinafter caprolactam) is a chemical intermediate primarily used in the production of nylon 6 fibers and resins.
About 90% of the world's production of caprolactam is based on the intermediate cyclohexanone, which is typically produced by the oxidation of cyclohexane. Caprolactam may also conventionally be produced by the partial hydrogenation of phenol. For the production of caprolactam by either process, cyclohexanone is reacted with a hydroxylamine to produce cyclohexanone oxime followed by a Beckmann rearrangement of the oxime using oleum to yield caprolactam. One disadvantage of the above-described conventional technology is that large amounts of ammonium sulfate—up to 4.5 tons/ton of caprolactam—are produced. Over many years much of the development work directed to manufacturing caprolactam from cyclohexanone has been focused on reducing or even eliminating this byproduct. For example, DSM's HPO Plus™ process (Hydroxylamine Phosphate Oxime), now believed to be used for the production of about 30% of the world's caprolactam, has substantially reduced the quantity of ammonium salt byproduct by as much as two thirds on a ton of salt/ton of product basis. More recently, Sumitomo has commercialized a process that eliminates the production of ammonium sulfate. The process employs an “ammoximation” reaction, whereby cyclohexane is reacted with ammonia and hydrogen peroxide in the presence of a catalyst, and a gas-phase Beckmann rearrangement. See, e.g., U.S. Pat. Nos. 6,265,574, 6,462,235 and 4,745,221. Significantly, one drawback of this process is the cost of hydrogen peroxide.
Other routes, developed primarily in the 1990s, sought to manufacture caprolactam from butadiene or adiponitrile. DSM, working first with DuPont and thereafter with Shell, developed the Altam process (see, e.g., WO 2002/083635), whereby butadiene and carbon monoxide are employed to make caprolactam without ammonium sulfate production. However, this process is still in the final phases of development and employs several complex catalytic reactions—carbonylation, hydroformylation, reductive amination, and cyclization. BASF and DuPont experimented with the production of caprolactam via adiponitrile, although it is not clear whether such processes are currently being practiced. See e.g., U.S. Pat. Nos. 6,372,939, 6,894,163, and 6,521,779; WO 2001/096294.
Toray has developed a photochemical process to convert cyclohexane into cyclohexanone oxime in the presence of nitrosyl chloride and hydrogen chloride, bypassing the use of cyclohexanone or the oximation step. Although this process may provide capital savings, the photochemical process demands significantly more power and the development of large scale photochemical reactors. See chapter devoted to caprolactam in Kirk-Othmer Encyclopedia of Chemical Technology 5th Edition, John Wiley and Sons 2001.
In addition to the above-mentioned shortcomings of the processes currently commercially employed and those being announced or developed as potentially viable alternatives, each of these processes suffers fundamentally from the increasing costs and volatility associated with the use of petroleum based feedstocks.
Thus, there remains a need for new, industrially scalable processes for the selective and commercially-meaningful conversion of a renewable feedstock, such as adipic acid derived from biorenewable materials, to caprolactam.